In a manufacturing process of a general silicon solar cell that constitutes a solar cell, a p-type silicon substrate is baked in a state where an n+ emitter is formed on a light-receiving surface side of the substrate according to a thermal diffusion using POCl3 and an Al paste is printed on a back surface of the substrate, thereby diffusing Al into the p-type silicon substrate to form a p+ back surface field (BSF) and form an n+pp+ structure. This process is a simple process in which electrode formation and diffusion processing can be performed at the same time by using Al, not only as an electrode but also as a diffusion source at the same time. Because an effect of suppressing a recombination of the back surface is high and a conversion efficiency of about 19% can be attained in the case of a monocrystalline silicon, this process is commonly used.
However, there is room for improvement in a long-wavelength reflectance of the back surface. A conversion efficiency of more than 20% can be realized by using an oxide film or the like for passivation, or by forming a back-surface reflective film or the like to enhance the reflectance of the back surface of the cell.
According to Non Patent Literature 1 , an output having a conversion efficiency of more than 20% is attained by forming a passivation film and a reflective film on the back surface. Patent Literature 1 also discloses a manufacturing process of a double-side light receiving solar cell.